The present invention generally relates to mobile radio communication systems, and more particularly to a mobile radio communication system which employs a multi-channel access system and is applicable to a mobile telephone or the like using radio zones.
Recently, there are increasing number of users of the mobile radio communication system such as the mobile telephone. Hence, it is desirable that the mobile radio communication system employs a handover control procedure which enables fast and efficient communication, so that the service and the frequency utilization efficiency are improved.
When a mobile station moves in the mobile radio communication system which uses the conventional multi-channel access (FDMA: frequency division multiplexing access) system and employs the radio zones, a handover is made when a called base station changes from an old base station to a new base station. In other words, the base station of an old radio zone in which the mobile station first exists and the base stations in neighboring radio zones measure the field intensity of a signal from the mobile station. A radio zone of the base station which receives the signal from the mobile station with the highest field intensity is regarded as a new radio zone in which the mobile station now exists.
FIG. 1 shows radio zones of an existing analog mobile telephone as an example of the conventional mobile radio communication system which uses the FDMA system and employs the radio zones. In FIG. 1, a control zone CZ surrounded by a bold solid line includes a plurality of radio zones which are under control of a single control station CS. In this example, the control zone CZ includes seven radio zones RZ1 through RZ7. A base station BSi is provided in each radio zone RZi, where i=1, 2, . . . , 7. FIG. 1 only shows a base station BS1 of the radio zone RZ1 and a base station BS2 of the radio zone RZ2. A mobile station MR exists within an arbitrary radio zone RZi within the control zone CZ. The mobile station MR may be a mobile telephone set equipped to an automobile or a portable telephone set which is hand carried by the user.
In FIG. 2, (A) and (B) respectively show control signals for controlling call-out and call-in, and in FIG. 3, (A) and (B) respectively show voice signals. As shown in FIGS. 2 and 3, a radio line uses a control channel C-CH and a voice channel V-CH which are not time-division-multiplexed. Hence, there is a going line from the base station BS1 or the like to the mobile station MR, and there is a returning line from the mobile station MR to the base station BS1 or the like.
Moreover, one control channel group f1 is assigned to one control zone CZ, and voice channel groups f8 through f14 are respectively assigned to the radio zones RZ1 through RZ7 so that the frequencies of the voice channels do not overlap among the radio zones RZ1 through RZ7 within the same control zone CZ.
FIG. 4 shows an example of a receiving system of the base station in the conventional mobile radio communication system. The receiving system includes a receiver 10 for receiving a voice channel signal, a receiver 20 exclusively for measuring the field intensity of a signal from the mobile station RM for the purpose of determining the radio zone in which the mobile station RM is located, and a controller 30 for controlling synthesizers 11 and 21 of the respective receivers 10 and 20 and the channel frequency in response to an instruction from the control station CS.
In the receiver 10, the received signal which passes through a bandpass filter 12 is mixed to a first local signal which has a frequency set by the synthesizer 11 in a mixer 13 and is converted into a signal having a first intermediate frequency. The output signal of the mixer 13 is passed through a bandpass filter 14a and is mixed to a second local signal which has a frequency set by a synthesizer 15 and is converted into a signal having a second intermediate frequency in a mixer 16. The output signal of the mixer 16 is passed through a bandpass filter 14b and is amplified by an intermediate frequency amplifier 17. A field intensity detector 18 detects the field intensity of the received signal based on an output signal of the intermediate frequency amplifier 17 and notifies the detected field intensity to the control station CS. A demodulator 19 demodulates the output signal of the intermediate frequency amplifier 17 and outputs an audio signal.
Similarly, in the receiver 20, the received signal which passes through a bandpass filter 22 is mixed to a first local signal which has a frequency set by the synthesizer 21 in a mixer 23 and is converted into a signal having a first intermediate frequency. The output signal of the mixer 23 is passed through a bandpass filter 24a and is mixed to a second local signal which has a frequency set by a synthesizer 25 and is converted into a signal having a second intermediate frequency in a mixer 26. The output signal of the mixer 26 is passed through a bandpass filter 24b and is amplified by an intermediate frequency amplifier 27. A field intensity detector 28 detects the field intensity of the received signal based on an output signal of the intermediate frequency amplifier 27 and notifies the detected field intensity to the control station CS.
FIG. 5 is a time chart for explaining a handover control procedure of the conventional mobile radio communication system which uses the base station shown in FIG. 4. For the sake of convenience, it is assumed that the mobile station MR first exists within the radio zone RZ1 as shown in FIG. 1 and communicates via the base station BS1 of the radio zone RZ1, and that the synthesizer 11 within the receiver 10 is tuned to the frequency of the mobile station MR in response to a frequency control signal from the controller 30 which receives an instruction from the control station CS.
In this case, when the base station BS1 is communicating, the field intensity detector 18 within the receiver 10 constantly monitors the field intensity of the signal from the mobile station MR in a step S11.
When the mobile station MR moves to the end of the radio zone RZ1 and moves from the radio zone RZ1 into the adjacent radio zone RZ2, the field intensity of the signal from the mobile station MR gradually decreases at the base station BS1. When the base station BS1 detects that the field intensity of the signal from the mobile station MR is less than a predetermined threshold value in a step S12, the base station BS1 notifies this information to the control station CS which controls the base station BS1 and the other neighboring base stations BS2 through BS7 within the control zone CZ in a step S13.
Responsive to the information from the base station BS1, the control station CS instructs all of the base stations BS1 through BS7 within the control zone CZ including the base station BS1 to measure the field intensity of the signal received from the mobile station MR in a step S14. The synthesizer 21 of the receiver 20 within each of the base stations BS2 through BS7 is tuned to the frequency of the mobile station MR, so that the field intensity of the signal received from the mobile station MR is output from the field intensity detector 28 within each of the base stations BS2 through BS7 within the control zone CZ in a step S15.
The base stations BS2 through BS7 respectively notify the measured field intensity of the signal received from the mobile station MR to the control station CS in a step S16.
The control station CS compares the field intensities notified from the base stations BS2 through BS7 and selects one of the base stations BS2 through BS7 which outputs the highest field intensity, and at the same time, selects an available channel (or free frequency) out of the voice channel groups f9 through f14 which are assigned to the base stations BS2 through BS7, in a step S17.
When it is assumed for the sake of convenience that the field intensity level notified from the base station BS2 is the highest, the control station CS notifies the base station BS1 of a handoff message and a new voice channel selected from the voice channel group f9 in a step S18 because the mobile station MR has now moved into the radio zone RZ2. This handoff message indicates that the communication between the communicating mobile station MR and the base station BS1 is ending. The base station BS1 transmits the received handoff message and the new voice channel to the mobile station MR and disconnects the transmission on the going line in a step S19.
At the same time, the control station CS notifies the handoff message and the new voice channel to the base station BS2 because the mobile station MR has now moved into the radio zone RZ2. Hence, the base station BS2 is put ready to transmit using the new voice channel.
The mobile station MR switches from the voice channel V-CH which is being used to the new voice channel in a step S20. In addition, the base station BS2 starts a communication with the mobile station MR using the new voice channel of the mobile station MR in a step S21.
Hence, the voice channel between the base station BS2 and the mobile station MR is established in the above described manner.
In the description given above, it is assumed for the sake of convenience that one mobile station MR moves within the control zone CZ. However, in actual practice, a plurality of mobile stations MR exist within the control zone CZ. Since the control zone CZ includes a plurality of radio zones RZ and each radio zone RZ has a radius in the order of several km, many mobile stations MR which require the handover may exist approximately at the same time.
In such a case, because the control station CS of the conventional system monitors the radio zones RZ1 through RZ7 within the control zone CZ, the control station CS cannot process the handovers which are simultaneously generated in a plurality of radio zones RZ within the control zone CZ. In addition, since each base station BS is only provided with a limited number of receivers 10 and 20, the base station BS cannot measure the field intensities of the signals which are simultaneously received from a plurality of mobile stations MR. As a result, there is a problem in that the handover process must wait until the turn of the mobile station MR comes.
Moreover, there is a problem in that the base station BS must be provided with the receiver 20 for measuring the field intensity of the signal from the mobile station MR in addition to the normal receiver 10 which is used for the communication.